An important consideration for a hair stylist preparing to treat a person's hair, is the existing condition of the hair before the treatment commences. For example, the compositions used for applying a permanent wave for a person's hair, may be quite different for undamaged hair which has previously had no more than mild treatment, as compared with hair which has been damaged by repeated bleaching. Highly skilled operators may evaluate the existing condition of the hair and select suitable treatments based on their experience with a variety of subjective visual and tactile examinations.
Such subjective evaluations of the quality of hair and selection of hair care products suitable for application to the hair, are subject to discrepancies due to the extent of experience and skill of the operator. Because of this the optimum hair care products may not be used, and in some cases selection of too harsh a hair care treatment may result in additional damage to hair that is already damaged.
For such reasons, there has long been a desire for objective techniques for evaluating the condition of hair or for determining the level of damage of hair, so that optimum hair care products can be identified. A fairly straightforward idea has been to measure the tensile properties of individual strands of hair. When a keratin fiber such as hair is subjected to tensile force or stress, it is elongated or strained before the force becomes large enough to break the hair. Dry virgin hair (i.e, undamaged) may stretch as much as 50% before breaking.
The elongation of a keratin fiber can be represented by a conventional force-elongation curve such as illustrated in FIG. 1, which is typical for wet virgin hair. Stress or force is plotted against strain or percentage elongation. The curve, which has a rather similar shape for all keratin fibers, consists of three regions. The Hookean region, which extends up to about 5% elongation, has a relatively small amount of elongation for a given increment of force, and much of the elongation is elastic. That is, the fiber elongation is generally linear with increasing stress, and upon release of the applied stress, the fiber will largely return to its original length. Through the yield region, which extends from about 5% to 30% elongation, there is appreciable elongation for a given increment of applied stress. The post-yield region extends from about 30% elongation to the break point of the fiber. In this region the slope of the force-elongation curve again increases.
The Hookean region represents the force required to overcome coulombic interactions between the side chains of the microfibrillar proteins, and width of the region, and the shape of the curve are affected by moisture content of the fiber, pH and temperature.
The yield region of the force-elongation curve is also quite sensitive to humidity and is associated with the transformation of the alpha helical segments of the microfibrillars into beta-sheets.
Beyond 30% elongation the fiber stiffens and considerably more force must be applied to complete the alpha-beta transformation and to overcome covalent keratin bonds, which are ruptured, leading to breakage of the fiber.
Analysis of the load-elongation or stress-strain curve from a tensile test of fibers, mainly in the yield region, has been the method of choice for assessing damage to keratin fibers. Over the years, several parameters derived from the curve, have been employed, with the 20% index being the most commonly used. This index is a ratio of the work required to elongate the fiber by 20% of its original length, after the fiber has been damaged to the work required to elongate the fiber by 20% before damage. Other investigators have used a 15% index or a 30% index. Any point falling within the yield region may be selected as an index. The advantage of using an index in the yield region, is that the force is relatively constant. Therefore, moderate differences in measurements of fiber elongation do not result in significant variations in the measured force.
A disadvantage of any of these indexes, however, is the force at any point on the force-elongation curve is directly proportional to the diameter of the fiber. Oriental hair, being coarser requires more force to stretch it, than does caucasian or negroid hair. Keratin fibers vary greatly in diameter even when taken from a single source. Human hair diameter can vary by as much as 100% on the same head.
Researchers have therefore resorted to either of two tedious techniques to overcome the diameter related variability of their selected index.
Some investigators test the same fiber before and after treatment. This is accomplished by stretching the fiber in water before treatment to the desired elongation, and then allowing that fiber to relax back to near the original condition. The assumption has been that the stretching does not affect the tensile properties of the fiber significantly. The relaxation process usually requires 24 hours, after which the fiber is treated (e.g., permed, bleached, straightened or dyed) and then retested. Despite the reliability of the technique, the time consuming relaxation process makes the technique inefficient and rather unappealing.
The other technique employed to overcome diameter effects is to test fibers of comparative diameter. This necessitates the aid of a microscope to scan fibers for homogeneity along the tested length and to sort the fibers according to size. This technique can be stressful to the investigator, inefficient when large numbers of samples are to be analyzed, and require additional costly equipment. Thus, even in a laboratory setting there is a great need for a simple fast technique, utilizing the properties of the force-elongation curve independently of fiber diameter and humidity.
The stress-strain curve techniques mentioned may be suitable for a laboratory environment where "before and after" testing is conducted. This is not of direct assistance to a salon operator who wishes to evaluate preexisting hair conditions before adopting a course of treatment. A before and after type evaluation is inapplicable. Even so, some measure of hair quality may be obtained by such an index, since the stress required for a given elongation of the hair tends to be reduced by damage to the internal structure of the hair. Salon operators may use a tensile testing machine and microscope to measure hair diameter and strength for evaluating a client's hair condition. Not only is such equipment costly, the time required for reliable testing is a precious commodity and few operators perform such objective testing.
Thus, there remains a desire for a simple and fast technique for evaluating hair damage in the context of a salon, so that the hair stylist may, before treatment commences, select the desired hair care product for a given client. Preferably, the technique is independent of other properties of the hair such as diameter, which is a tedious measurement. It is also desirable that the technique be independent of humidity which can have an appreciable effect on strength properties of hair.
There are other keratin fibers where a fast, simple and reliable test for condition of the fibers is useful. For example, buyers and users of wool presently evaluate quality by subjective visual and tactile factors. It is desirable to provide objective measures of wool quality which can be used by skilled or relatively unskilled persons with reliability.